1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to reduction of the peak power and speckle contrast for bright field and dark field inspection applications.
2. Description of the Related Art
Many optical systems are designed to produce images such as an inspection system for a partially fabricated integrated circuit or a photomask. Techniques and apparatus for inspecting circuits or photomasks are well known in the art and are embodied in various commercial products such as many of those available from KLA-Tencor Corporation of San Jose, Calif. Most optical imaging systems use a continuous illumination source. However, many times pulsed illumination sources are preferred or are the only sources available. This is especially true in the DUV spectral region below 250 nm where very few high brightness illumination sources exist that are not pulsed. Common examples are excimer lasers used in the photolithography process for manufacturing semiconductor devices.
If a pulsed illumination source is used, the optical imaging system must contend with the nature of pulsed illumination. This is especially true when inspecting integrated circuits or photomasks. Pulsed illumination typically suffers from two major problems. First, the peak power of the illumination transmitted from the illumination source can be very high and potentially damage elements in the optical system or the object being inspected. Second, the light energy can suffer from “speckle” or random intensity distribution of light due to interference effects. This is especially true for laser light sources.
Further, instances occur wherein a higher repetition rate source is not available. A system or device for turning a high repetition rate laser, such as a mode locked laser, into a virtually continuous source would be very useful in these situations.
One prior apparatus for reducing the peak power of a pulsed laser is U.S. Pat. No. 5,309,456 by Horton. The Horton design uses one mirror and one beamsplitter to split a single laser pulse into multiple pulses. The multiple pulses are then delayed with respect to each other using reflective optical delay schemes. Several drawbacks exist for this approach. First, the pulse-to-pulse uniformity is highly dependent on the quality of the mirror and beamsplitter used to form the multiple pulses as well as losses in the optical delay schemes. To maintain uniform pulses this system requires 100% reflective mirrors, 50% reflective and 50% transmissive beam splitters with no absorption, and perfect AR coatings with 100% transmission. Any deviations from this will cause an energy variation between the pulses. For example, consider the effects of imperfect optics on a system that generates 16 pulses. If the beamsplitter transmission is 49% and the reflectivity is 51%, the energy variation between the pulses will be 16%. In addition, if the mirror has a reflectivity of 99% it will cause an additional energy variation between the pulses of 3%. Another limitation of the Horton design is that it is not well suited for the DUV-VUV spectral range. Reflective coatings are much less efficient in this range and can cause large losses. These losses will contribute to pulse-to-pulse nonuniformity and a reduction in the efficiency of the peak power reduction scheme. For example, a reflective optical delay scheme with a 1 m long cavity, using mirrors with 99% reflectivity, and an optical delay of 10 meters will have a loss of 10%. Similarily, a reflective optical delay scheme with an optical delay of 20 m, 40 m, and 80 m will have losses of 18%, 33%, and 55% respectively. If these delay paths are used in a system to generate 16 pulses, assuming perfect 50% beam splitters and a perfectly reflecting mirror, the lowest energy pulse will be only 22% of the highest energy pulse. An additional limitation of the Horton design is that it uses a single mirror and beamsplitter to generate the multiple pulses. This optical setup is not flexible and inhibits compensation of different losses for each delay path. In addition, this scheme offers no solution for dealing with the effects of speckle.
With respect to speckle problems, two primary techniques have been used in the past to reduce the contrast of speckle in a single laser pulse. The first technique employs two rotating diffusers to create multiple speckle patterns during a single pulse. This technique relies on the relative motion of the two rotating diffusers to produce uncorrrelated speckle patterns. This technique has several major disadvantages. First, the diffusers must rotate at a high at a high rate of speed to produce smoothing within a pulse. For a typical pulse of 20 ns, only a limited number of uncorrelated speckle patterns can be produced. Also, losses from diffusers are typically very high. A typical transmission for such a diffuser is 40%. The diffusers in combination then have a transmission of only 16%. In addition, rotating diffusers can be a source of vibration that can effect the image quality of the system. The second technique uses two diffraction gratings and an electro-optic modulator to produce speckle smoothing within a single pulse. This scheme was developed to minimize speckle problems for laser fusion systems. This technique has several limitations including large size and very high cost. In addition, electro optic modulators operating at high bandwidths in the DUV and VUV ranges are not available.
It is therefore an object of the current invention to provide a system or arrangement that can reduce the peak power of a laser pulse emanating from an energy source.
It is another object of the current invention to provide an illumination solution that does not suffer excessive losses due to mirrors, beamsplitters, and optical delay lines but that can produce substantially uniform pulses.
It is a further object of the current invention to provide an illumination solution that can be readily reconfigured while producing optical delays with minimum optical losses, particularly in the DUV-VUV spectral region.
It is still a further object of the current invention to provide an illumination solution, having reduced speckle contrast for a single energy pulse.
It is yet a further object of the current invention to provide for speckle contrast reduction in an illumination system preferably employing a pulsed illumination source wherein said speckle contrast reduction may be employed in combination with other speckle reduction schemes to further reduce the speckle contrast of a single pulse.
It is yet another object of the current invention to effectively increase the repetition rate of a pulsed source and further to achieve quasi-continuous operation from a high repetition rate source.